High-purity polycrystalline silicon (polysilicon) serves as a starting material for production of monocrystalline silicon for semiconductors by the Czochralski (CZ) or zone melting (FZ) processes, and for production of mono- or polycrystalline silicon by various pulling and casting processes for production of solar cells for photovoltaics.
Polysilicon is typically produced by means of the Siemens process. This involves passing a reaction gas comprising one or more silicon-containing components and optionally hydrogen into a reactor comprising support bodies heated by direct passage of current, silicon being deposited in solid form on the support bodies. The silicon-containing components used are preferably silane (SiH4), monochlorosilane (SiH3Cl), dichlorosilane (SiH2Cl2), trichlorosilane (SiHCl3), tetrachlorosilane (SiCl4) or mixtures of the substances mentioned.
The Siemens process is typically conducted in a deposition reactor (also called “Siemens reactor”). In the most common embodiment, the reactor comprises a metallic base plate and a coolable bell-shaped casing placed onto the base plate so as to form a reaction space within the bell-shaped casing. The base plate is provided with one or more gas inlet orifices and one or more offgas orifices for the departing reaction gases, and with holders which help to hold the support bodies in the reaction space and supply them with electrical current.
Each support body usually consists of two thin filament rods and a bridge which connects generally adjacent rods at their free ends. The filament rods are most commonly manufactured from mono- or polycrystalline silicon; less commonly, metals, alloys or carbon are used. The filament rods are inserted vertically into electrodes present at the reactor base, through which they are connected to the power supply. High-purity polysilicon is deposited on the heated filament rods and the horizontal bridge, as a result of which the diameter thereof increases with time. Once the desired diameter has been attained, the process is ended.
Modern reactors may contain up to 100 filament rods or more. A high number of rods enables high reactor productivity and reduces the specific energy consumption, since the energy losses are reduced, for example, by the radiation to the cold reactor wall.
In prior art reactors, the rods in the reactor are frequently arranged in concentric circles around the center of the base plate. The number of circles depends on how many rods the reactor accommodates.
U.S. Pat. No. 4,681,652 A discloses reactors having 5, 6, 10, 12 and 20 rods, the rods each being positioned in two concentric circles according to the following schemes: 1+4, 2+4, 4+6, 4+8, 8+12 (the first number gives the number of rods in the inner circle, the second number the number of rods in the outer circle).
US 2010/0043972 A1 discloses a reactor having 48 rods, the rods being distributed in three circles: 8+16+24. The bridges connect the rods in pairs within the rod circles, such that the rod pairs or support bodies formed likewise form three concentric circles.
Since the bridge length is small compared to the rod length, the position of the bridges generally does not play any significant role.
Usually, two adjacent rods from a circle are connected by means of a bridge (as in the abovementioned US 2010/0043972 A1).
Also known are designs where some bridges are aligned radially and two rods from the different circles are connected to one another.
U.S. Pat. No. 3,011,877 A describes a reactor in which the rods are inclined and are in contact at their free ends, such that no bridge at all is necessary. Also outlined therein is a means of connecting three rods, the energy being supplied in this case with a three-phase alternating current.
With increasing number of rods, the number of circles on which the rods are arranged generally also increases.
US 2009/0136408 A1 discloses a reactor with 98 rods, the rods being distributed in five circles (6+12+22+26+32).
Gas inlet orifices (for the injection of the fresh reaction gas) are usually positioned in the middle of the reactor (i.e. within the inner rod circle) and/or between the rod circles.
Offgas orifices are generally likewise provided in the middle of the reactor (i.e. within the inner rod circle) and/or between the outer rod circle and the reactor wall.
Also known are designs in which the offgas leaves the reactor via orifices in the upper reactor section.
Sometimes, cooling elements are introduced into the reactor space. They serve to reduce the gas space temperature and may be configured and positioned in different ways.
In general, such cooling elements are designed as cooled shields around rods and/or bridges such that the rods are encapsulated thereby (see, for example, EP 0 536 394 A1).
Also known are embodiments with a cooling finger inserted into the reactor from above (DE 195 02 865 A1) or with a cooled tube introduced in the middle (DE 10 2009 003 368 B3), which is designed as an extension of an offgas orifice.
In some cases, a stationary heating element which is inserted only for ignition of the filament rods is present in the reactor space. Such a heating element, in the case of reactors with several rod circles, is usually in the middle of the reactor, i.e. is positioned within the innermost rod circle (see, for example, US 2009/0136408 A1 or GB 1131462 A).
In the production of thick polycrystalline silicon rods (with diameter >100 mm) in the prior art Siemens reactor, it is a relatively frequent observation that the rods have regions with a very rough surface (“popcorn”). These rough regions have to be removed from the rest of the material and be sold at much lower prices than the rest of the silicon rod.
FIG. 5 shows regions of silicon rods with a smooth surface (FIG. 5 A) and with a popcorn surface (FIG. 5 B).
By adjusting the process parameters (for example reducing the temperature of the rods), the proportion of the popcorn material can be reduced (see U.S. Pat. No. 5,904,981 A1).
Such changes, however, lead to the effect that the process runs more slowly and hence the yield is reduced, which worsens the economic viability.
The object was therefore that of producing polycrystalline rods from high-purity silicon with a low proportion of rough surfaces in a more economically viable manner.
This object is achieved by the present invention.